It's a Dark Squiglet. They live in the Flopside Pit of 100 Trials... Max HP is 20. Attack is 8. It hops when it sees an enemy, then spits rocks... It spits quickly, so jump as soon as you see it hop. Some people think that little hop is quite cute...

Dark Squiglets are enemies in Super Paper Mario. They are the shadowy, featureless, counterparts of the Squiglets, although they also throw rocks, like Squiglets' other relatives. Just like all dark enemies, Dark Squiglets can only be found in the Flopside Pit of 100 Trials. They have a dangerously high attack power and have an HP of 20. When they spit out rocks, the damage is 8, but if a player bumps into one, the damage is only 1. Like their relatives, Dark Squiglets perform a small jump before attacking.